As a semiconductor layer in display devices such as liquid crystal displays or electroluminescent displays, amorphous silicon or low temperature polysilicon has been used widely. However, various oxide semiconductor materials are now developed against the backdrop of the need for displays to have a larger screen, a higher definition, a lower power consumption, etc.
Oxide semiconductor materials are composed mainly of indium, gallium, zinc and tin, and oxides of various composition are now studied, as exemplified by indium gallium zinc oxide (IGZO), indium gallium oxide (IGO), indium tin zinc oxide (ITZO), indium gallium zinc tin oxide (IGZTO), indium gallium zinc silicon oxide (IGZSO), gallium zinc oxide (GZO), zinc tin oxide (ZTO) and so on. Among them, oxides containing indium and gallium (e.g., IGZO and IGO) are particularly well studied.
An oxide containing at least indium and gallium, such as an oxide consisting of indium, gallium and oxygen or an oxide consisting of indium, gallium, zinc and oxygen, is formed on a substrate (e.g., glass) by means of film formation processes such as sputtering techniques. Subsequently, a resist or the like is used as a mask and the oxide formed on the substrate is etched to form an electrode pattern. This etching process includes wet and dry processes, and an etchant is used in wet processes.
Oxides containing at least indium and gallium, such as an oxide consisting of indium, gallium and oxygen, as well as an oxide consisting of indium, gallium, zinc and oxygen, are generally known to be soluble in acids. In the case of using these oxides to form semiconductor layers in display devices such as liquid crystal displays, an etchant is required to have the following performance characteristics.    (1) Upon oxide dissolution in the etchant, no precipitate occurs.    (2) Upon oxide dissolution in the etchant, a reduction in the etching rate is small.    (3) The etchant has a preferred etching rate.    (4) The etchant gives few etching residues.    (5) The etchant causes no corrosion in elements such as wiring materials.
With the progress of etching of oxides, the oxide concentration in the etchant is increased, but the etchant remains stable because its etching rate is only slightly affected upon oxide dissolution. This property means that the amount of etched oxides per fixed volume of etchant can be increased, and it is very important in industrial production to conduct etching of semiconductor layers.
Moreover, the etching rate is desirably 10 to 1000 nm/min, more preferably 20 to 200 nm/min, and even more preferably 50 to 100 nm/min. At an etching rate of 10 to 1000 nm/min, the production efficiency can be maintained and etching operations can be conducted stably.
In cases where etching residues having conductivity are left after etching, these residues are not favorable because they may cause leakage current between electrodes. Alternatively, in cases where etching residues having no conductivity are present, these residues may be responsible for poor wiring, void formation, poor adhesion and so on in the subsequent steps.
Wiring materials include copper (Cu), aluminum (Al), molybdenum (Mo) and titanium (Ti), and there may be a possibility that an etchant will contact with these wiring materials during oxide etching. Thus, a preferred etchant is less corrosive to wiring materials. More specifically, the etching rate to the wiring materials is desirably 3 nm/min or less, more preferably 2 nm/min or less, and particularly preferably 1 nm/min or less.
When an etchant containing oxalic acid is used for etching of transparent conductive films composed of indium oxide as a major component, there arises a problem that a salt formed between oxalic acid and indium will be precipitated as a solid matter with the progress of etching. The appearance of this precipitate would be due to the presence of excess indium which forms a salt with oxalic acid and appears as a precipitate because the soluble concentration of indium is around 200 ppm in commonly used etchants containing oxalic acid. The precipitation of such a solid matter is fatal in the production processes of electronic components where even particles smaller than 1 μm will cause a problem. Moreover, this precipitate will clog filters which are provided for etchant circulation, and high costs may be required for their replacement. For this reason, even when an etchant still has sufficient performance, the etchant should be replaced with fresh one before precipitation of this salt, as a result of which the etchant is made to have a shorter period for use.
Patent Document 1 (WO2008/32728) describes that when a composition containing (a) oxalic acid, (b) a naphthalenesulfonic acid condensate or a salt thereof, (c) at least one of hydrochloric acid, sulfuric acid, a water-soluble amine and salts thereof, and (d) water is used for etching of indium tin oxide films (ITO films), no residue occurs and it is further possible to prevent precipitation of a salt between oxalic acid and indium.
Patent Document 2 (JP 2010-45253 A) describes that when using an etchant for transparent conductive films, which comprises oxalic acid and an alkaline compound (except for triethanolamine), it is possible to effectively prevent precipitation of indium oxalate crystals even at a high indium concentration in the etchant during the etching processes of transparent conductive films such as ITO films or IZO films.
Patent Document 3 (JP 2009-218513 A) discloses an oxalic acid-free etchant, which comprises, e.g., sulfuric acid and an anionic surfactant having a hydrocarbon group containing 12 or more carbon atoms as major components. This etchant is regarded as having a longer life as an etchant because of exerting a good ability for residue removal and a high ability for indium dissolution to thereby prevent solid matter precipitation when used for etching of amorphous indium oxide (ITO)-based films.
Patent Document 4 (JP 2006-77241 A) discloses an etchant for indium oxide-based transparent conductive films such as ITO films or IZO films, which comprises sulfuric acid as a main oxidizer and phosphoric acid, nitric acid, acetic acid or the like as an auxiliary oxidizer.
Patent Document 5 (JP 2000-8184 A) discloses an etching process using an etchant containing sulfuric acid, nitric acid and a buffer (e.g., acetate, phosphate, oxalate, formate and citrate buffers) for etching of a multi-layer conductive film constituted by laminating a silver-based thin film and a transparent oxide thin film composed of indium oxide as a major component.
Patent Document 6 (JP 2008-41695 A) discloses an oxide etching process, which is characterized by involving a step where an amorphous oxide layer comprising indium and at least one selected from gallium or zinc is etched with an etchant comprising any one of acetic acid, citric acid, hydrochloric acid or perchloric acid.
Patent Document 7 (JP 2007-317856 A) describes that when an etchant composed of an aqueous solution comprising one or more compounds selected from the group consisting of organic acids (e.g., citric acid, aconitic acid) and ammonium salts thereof is used for etching of transparent conductive films comprising zinc oxide as a major component, no residue occurs, an adequate etching rate is achieved and the etching performance is stable against zinc dissolution.